In LCDs, as in various types of displays in general, the contrast performance of the display is one of the key features affecting the user experience of the display. Primarily, contrast refers to the emitted intensity difference between bright and dark pixels or regions of the display. Typically, especially the darkness of the pixels or regions desired to represent black should emit as low light as possible. The term high dynamic range HDR is often used to refer to a display's functionality related to extremely high difference between the maximum and minimum brightness of the pixels or regions of the display.
In an LCD, the intensity of the light emitted by a pixel or region is generally determined by the backlight illumination intensity and the transmittance of the liquid crystal (LC) module at that pixel/region.
Conventionally, backlight arrangements with substantially uniform illumination intensity at the LC module have been used. Then, the contrast performance of the display panel is determined solely by the adjustability of the transmittance through the LC module. In practice, the LC modules never achieve complete opacity, so with uniform backlight intensity, the deepest dark achievable is always somewhat greyish.
In direct-lit backlight configurations where the light sources, typically LEDs, are located behind or below the LC module, attempts to relieve the above problem have been made based on spatially non-uniform control of the illuminating LED array producing the backlight illumination. In such configurations, the LED panel is divided into sections, the LEDs of which are controlled separately from the other sections. Then, LEDs of the sections illuminating dark or dim regions of the image to be displayed may be controlled so as to emit less or no light, whereas LEDs of the sections corresponding brighter regions of the image are controlled to emit more light. Thereby, the dynamic range of the display can be increased to some extent.
There are many applications, such as various types of mobile devices, where the device configuration with typically relatively low thickness does not allow the use of a direct-lit backlight arrangement, or at least would benefit from the use of an edge-lit backlight configuration. On the other hand, the spatial brightness control by means of separate control of LED segments may be insufficient in some applications. Even with only one LED in one segment, the area of the LC panel illuminated by a single segment may be undesirable large.